The present invention relates to prodrugs for antioxidant therapy. In particular, the present invention relates to estrogen-related steroidal quinols and their use as prodrugs for phenolic A-ring estrogens and estrogen analogs.
Mammalian cells are continuously exposed to reactive oxygen species (ROS) such as lipid peroxyl, oxoperoxinitrate, superoxide (O2.−), hydrogen peroxide (H2O2), hydroxyl radical (OH.), and singlet oxygen (1O2). In vivo, these reactive oxygen intermediates are generated by cells in response to aerobic metabolism, catabolism of drugs and other xenobiotics, ultraviolet and x-ray radiation, and the respiratory burst of phagocytic cells (such as white blood cells) to kill invading bacteria such as those introduced through wounds. Hydrogen peroxide, for example, is produced during respiration of most living organisms especially by stressed and injured cells.
ROS, when present in excess, can be detrimental to cells. If the cellular balance of the level of oxidizing species (i.e., reactive oxygen species and reactive nitrogen species) is not restored, several pathological processes are elicited, including DNA damage, lipid peroxidation, loss of intracellular calcium homeostasis, and alteration in cellular signaling and metabolic pathways. Oxidative stress causes cellular damage, resulting in alteration of the redox state (i.e., depletion of nucleotide coenzymes and disturbance of sulfhydryl-containing enzymes), and saturation and destruction of the antioxidant defense and DNA repair system.
For example, excess hydrogen peroxide can react with DNA to cause backbone breakage, produce mutations, and alter and liberate bases. Such oxidative biochemical injury can result in the loss of cellular membrane integrity, reduced enzyme activity, changes in transport kinetics, changes in membrane lipid content, and leakage of potassium ions, amino acids, and other cellular material.
Another example of the ability of ROS to injure cells is lipid peroxidation, which involves the oxidative degradation of unsaturated lipids. Lipid peroxidation is highly injurious to membrane structure and function and can cause numerous cytopathological effects. Researchers propose that atherosclerosis and its deadly effects of heart attack and stroke develop in relationship to the oxidation modification of low-density lipoproteins (LDL) carrying cholesterol in the blood. It is theorized that free radicals generated by the body's own immune cells oxidize LDL, which are taken up by cells of the vascular intima initiating the atherosclerosis lesion.
Thus, oxidative stress has been associated with a variety of diseases and disorders, including aging and neuronal cell death (Jenner, P., “Oxidative damage in neurodegenerative disease,” Lancet, 344, 796–798 (1994)). For example, oxidative stress is associated with the pathology of numerous neurodegenerative diseases and conditions including, but not limited to, Alzheimer's disease, diabetic peripheral neuropathy, multiple sclerosis, amyotrophic lateral sclerosis, Huntington's disease, and Parkinson's disease.
The brain is a specialized organ that concentrates metals necessary for normal neurological functions. However, trauma, ischemia, and many other insults of neuropathological origin are known to release protein bound metal ions such as iron from damaged cells. The release of metal ions increases oxidative stress in the central nervous system (CNS) by promoting the generation of ROS.
Antioxidants have been shown to inhibit damage associated with ROS. For example, pyruvate and other alpha-ketoacids have been reported to react rapidly and stoichiometrically with hydrogen peroxide to protect cells from cytolytic effects (O'Donnell-Tormey et al., “Secretion of pyruvate. An antioxidant defense of mammalian cells,” J Exp. Med., 165, 500–514 (1987)). Selegiline, which may act as an antioxidant since it inhibits oxidative deamination, has been found to delay the onset of Parkinson's disease (Youdim, M. B. H., and Riederer, P., “Understanding Parkinson's disease,” Scientific American January, 52–59 (1997)). Antioxidant therapy has been demonstrated to slow the rate of motor decline early in the course of Huntington's disease (Peyser C. E., et al., “Trial of d-alpha-tocopherol in Huntington's disease,” Am J. Psychiatry, 152, 1771–1775 (1995)). PROBUCOL (4,4′-[(1-methylethylidene)bis(thio)]bis[2, 6-bis(1,1-dimethylethyl)] (Lorelco, Marion Merrell Dow), an antioxidant, is effective in reducing the rate of restenosis after balloon coronary angioplasty (Tardif, J. C. et al., “Probucol and multivitamins in the prevention of restenosis after coronary angioplasty. Multivitamins and Probucol Study Group,” New Engl. J. Med. 337, 365–372 (1997)).
Unfortunately, many antioxidants are fat-soluble and restricted in usage because of low water solubility. Those antioxidants that are water-soluble and less restricted in usage, such as vitamin C, may act as a pro-oxidant, i.e. an oxidation promoter in the presence of a metallic ion, and have the drawback of promoting lipid peroxidation under certain conditions. Uric acid is also water-soluble, but when accumulated in vivo, may generate unpleasant side effects such as gout or renal calculus. PROBUCOL demonstrates little bioavailability.
Estrogens have been recognized as antioxidants and potent neuroprotective agents. Their antioxidant action is believed to be due to their ability to scavenge free radicals that cause neuronal cell death. Estrogens, like the highly potent endogenous antioxidant vitamin E (α-Tocopherol), have a phenolic moiety considered a quintessential feature in achieving protection against oxidative stress. Studies, however, have concluded that the potency of the estrogen estradiol as a phenolic antioxidant on inhibiting iron-induced lipid peroxidation to be greater than that of vitamin E despite the extremely low overall concentration of estrogens compared to vitamin E. In addition, the OH- bond dissociation energy (BDE) of estradiol is greater than that of vitamin E, which would imply that vitamin E is a stronger deactivator of oxyradicals than estrogen. Antioxidant potency is generally determined not only by the chemical reactivity toward ROS, but also by the mobility and/or distribution of the molecule in the microenvironment and the fate of the antioxidant derived radicals (i.e., the dynamics of antioxidant action). Therefore, lipophilic estrogens may act in vivo as highly localized antioxidants despite their small bulk levels due to membrane binding affinity and high concentrations near the loci of activity.
Estrogen replacement therapy (ERT) has been associated with numerous health benefits, including alleviation of menopausal symptoms, bone and cardiovascular protection, reduction in the incidence of Alzheimer's disease, and improvement in cognitive functions, Parkinson's disease, and the outcome of stroke. These diverse activities of estrogens may be related to their cytoprotective effects and antioxidant abilities. The neuroprotective effect of estrogens against numerous toxic insults including oxidative stress has been extensively investigated in vivo and in vitro in several types of neuronal cells. There is mounting evidence that estrogens exert their neuroprotective effect against oxidative stress by suppressing the neurotoxic stimuli via their direct radical-scavenging activity.
Estrogens are degraded in the intestinal tract and rapidly metabolized by the liver. Specifically, estrogens undergo enterohepatic recirculation via sulfate and glucuronide conjugation in the liver, biliary secretion of conjugates into the intestine, and hydrolysis in the gut followed by reabsorption. The estrogen concentration encountered by the liver is generally four-fold to five-fold greater than estrogen levels in peripheral blood (the “first pass effect”). Administration of oral estrogens present high levels to the liver and may lead to an undesirable increase in the production of certain coagulation factors and renin substrates by the liver. Therefore, there is a need for therapeutic agents that are pharmaceutically effective at those regions where they are required.
High doses of estrogen have been demonstrated as having achieved an anti-oxidant effect in vitro. It has been demonstrated that the most biologically active estrogen, 17β-estradiol, is a potent antioxidant and has neuroprotective activity; however, the mechanism of action is still unclear. Such doses, even if effective on cells in vivo, would have limited utility in treating conditions associated with oxidative stress because of associated problems with toxicity, increased incidence of some forms of cancer, and feminizing effects on men. Thus, the usefulness of such a method of treatment is quite limited.
Therefore, a need exists for compositions and methods of administering estrogen-related free-radical scavengers or antioxidants to tissues demonstrating alterations in oxidative conditions. In particular, there is a need for compositions and methods that can provide therapeutic benefits to subjects suffering from neurodegenerative diseases associated with oxidative stress. Furthermore, there exists a need for a therapeutically effective estrogen compound that retains its therapeutic activity without any associated sex-related side effects.